MLLW depth - EPSG:5866

WKT

VERTCS["MLLW_Depth", 
	VDATUM["Mean_Lower_Low_Water"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", -1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["MLLW_Depth", 
	VDATUM["Mean_Lower_Low_Water"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", down, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5866"]]

Data from @esri/proj-codes

{
  "wkid": 5866,
  "latestWkid": 5866,
  "macro": "PE_VCS_MLLW_D",
  "name": "MLLW_Depth",
  "wkt": "VERTCS[\"MLLW_Depth\",VDATUM[\"Mean_Lower_Low_Water\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",-1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"MLLW_Depth\",VDATUM[\"Mean_Lower_Low_Water\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",down,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5866\"]]",
  "description": "MLLW depth",
  "authority": "EPSG",
  "version": "7.12(10.2.0)",
  "deprecated": "no",
  "areaname": "World",
  "extent": {
    "slat": -90,
    "nlat": 90,
    "llon": -180,
    "rlon": 180
  }
}